The present invention generally relates to the testing and evaluation of mechanical systems and, more particularly, to a broadband energy resonance test for noise evaluation of mechanical systems such as generators.
In operation, mechanical systems such as generators produce noise. The xe2x80x9cnoise signaturexe2x80x9d of these mechanical systems generally includes components that are attributable to many different aspects of the system operation. In the case of generators, the noise signature may include components that are attributable to, for example, a loose core and keybar rattle.
FIG. 1 shows a cross-section of a simplified conventional dynamoelectric generator 10 shown in U.S. Pat. No. 4,837,471. Generator 10 includes a stator 12 that has a generally annular shape. The outermost portion of generator 10 and stator 12 is the generator frame 13.
The stator core 14 is built up by stacking a large number of stator lamination sectors 16 together in a sandwich-like relationship. Stator lamination sectors 16 are attached to the stator using keybars. A rotor 20 is rotatably mounted in a cylindrical opening 22 formed along the central axis 24 of stator 12. That is, rotor 20 is coaxially positioned within stator 12 such that rotor 20 may be freely turned with respect to stator 12. Rotor 20 and stator 12 include respective windings (not shown). External excitation power is generally supplied to the rotor field windings via slip rings (not shown) coupled to an external power source. Thus, when mechanical energy is applied to rotor 20 to cause rotor 20 to spin on its axis 24, a moving magnetic field is generated which rotates at the same rate as rotor 20. This moving magnetic field cuts across the stator windings thus causing an electric current to be generated with the stator field windings.
FIG. 2 discloses a stator 30 which includes a generally annular stator frame 40, formed by outer wrapper 42 and a plurality of web plates arranged in annular fashion as indicated in the portion of stator 30 shown in perspective. Keybars 50 include opposed ends, one of which is shown as 50A. A plurality of keybars 50 are situated in respective holes 52 which are machined in the radially inner edge 56 of web plates 44. Keybars 50 are used to attach lamination sectors 58 to web plates 44. Each keybar has a cylindrical portion 60 that is situated within holes 52 and a dovetail portion 62 that extends radially inward from cylindrical portion 60. The dovetail portions 62 of keybars 50 mate with respective dovetail slots 64 in the radially outer curved edge 66 of stator core lamination sectors 58. The portion of stator 30 shown in FIGS. 2 and 3 includes one of a plurality of stator slots 70 which contain conventional current carrying conductors 72. Stator conductors 72 are held in slots 70 by a conventional dovetail retaining bar 74.
Generators with loose cores emit a single frequency transformer-type humming sound that, when harmonically filled, can be erroneously evaluated as keybar rattle caused by impacting of the dovetail to core iron (punchings). More specifically, keybar rattle is the result of a loose interface between the core iron (punchings) and the sections of the keybar designed to lock the punchings to the bar. Because the noise signature of a mechanical system such as a generator results from different factors, an intelligent and consistent condition-based acquisition and evaluation of noise data is desirable. In particular, it is important to differentiate between noise caused by different factors (e.g., loose core and keybar rattle) and to determine whether the noise data (or particular components thereof such as noise from keybar rattle) is indicative of any condition requiring repair and whether the repair must be done immediately or may be done at some later date.
It is therefore seen to be desirable to accurately determine and evaluate the relative condition of mechanical system designs at different operational conditions.
During operation, mechanical systems emit a signature noise characteristic ranging from single frequency to either a random or harmonically filled spectrum. The method used in a preferred embodiment of the present invention is much like the differentiation (a specific frequency) and the evaluation (harmonic content) of a specific toned musical instrument within a fifty-piece band. The method is based on the theory that the accumulation of harmonic content within a generator noise spectrum creates a consistent signature index number relating to specific mechanical conditions within the generator. The harmonic peak picking and the subtracting of strategically averaged random noise effects create an intelligible representation of specific mechanical system conditions.
In accordance with a preferred embodiment of the present invention, a noise index for a mechanical system is generated by acquiring noise data over a predetermined frequency range. A fundamental harmonic frequency is chosen and this fundamental harmonic frequency is used to mark harmonic and sub-harmonic data bins. The amplitude of the harmonic and sub-harmonic data bins are recorded and a plurality of sub-harmonic bins equally offset from the center of each harmonic frequency are averaged, the average being subtracted from the preceding harmonic level. The results are accumulated as harmonic content levels indicative of a condition of the mechanical system. The results can be used to determine those conditions requiring repair as well as whether the repair must be done immediately or may be done at some later date.